RFC 4288:Media Type Specifications and Registratio...
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Macintosh File Type

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10. Registration Template
... File extension(s): Macintosh file type code(s): Person & email address ...
... Some discussion of Macintosh file type codes and their purpose can be found in [MacOSFileTypes]. Additionally, please refrain from writing ...
... "none" or anything similar when no file extension or Macintosh file type is specified, lest "none" be confused with an actual code value. ...

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